diff --git a/bip-0138.md b/bip-0138.md
index b6546220..1cd0e1da 100644
--- a/bip-0138.md
+++ b/bip-0138.md
@@ -138,7 +138,7 @@ Implementations:
 - MUST exclude every other form (literal pubkeys, bare xpubs, ...) from
   $\{p_1, \dots, p_n\}$.
 - MUST refuse to encode a backup if the resulting set is empty.
-- SHOULD make the user aware of each excluded expression, since the
+- MUST make the user aware of each excluded expression, since the
   cosigner holding that key will be unable to decrypt the backup with
   their key.